1. Field of the Invention
The present invention relates to the inspection and monitoring of moving materials, as for instance on a conveyor or the like, during the production process for thickness, profile or other quality control data. More particularly the invention relates to a method and apparatus for providing a reference standard which may be used to compensate for displacement of the elements of a scanning frame or fixture due to thermal or other stress. The present invention contemplates the use of a reference wire or wires which extend between the scanning frame members and which are maintained under constant tension and hence constant configuration, regardless of changes in the ambient temperature or temperatures of the product being scanned.
2. Description of the Prior Art
Prior art methods and apparatus for scanning a moving piece of material such as a web or panel are well known and are used for quality control to monitor for example the thickness, width or other dimension of the product. Some systems are also designed for monitoring properties such as surface texture, density, mass and quality of the product. Although some systems utilize devices which actually contact the material surface, most systems utilize one of the many known non-contact methods with sensors employing a wide range of energy beams or waves including light, sound, electrical current, and particle transmission or absorption.
Regardless of the type of sensor utilized, initial calibration and constant adjustment is usually necessary in order to maintain quality monitoring and accurate measurements. The character of the initial calibration and the need for adjustment or recalibration usually depends on the type of material being monitored, the design of the fixture and the scanning environment which may introduce factors such as physical vibrations or extreme variations in temperature. The need for adjustment and recalibration will also be dependent somewhat upon the nature and sophistication of the sensors themselves. U.S. Pat. No. 5,327,770 to Hindle is an example of prior art scanning systems wherein a sensor or an array of sensors is mounted for transverse movement relative to the direction of flow of material for sampling a web or like. In the Hindle system a primary scanner scans off the edge of the moving product and scans a sample of the same material of known characteristics for periodically updating its calibration. The primary scanner then uses a correction signal to recalibrate secondary scanners. The specification of the Hindle patent also discusses a prior art embodiment, GB-A1,378,303 which addresses the problem of thermal stress correction by splitting a light beam so that a portion is directed into a stationary detector while another passes through a moving web. An attempt is made to correct the errors caused by thermal stress and bending of supporting structures. The reference signal is then used for measurement purposes only and not calibration.
Other prior art devices teach various methods for calibrating and updating energy beam sensors. U.S. Pat. No. 3,750,461 to Felix for instance is a system wherein the ultrasonic sensors are calibrated with a separate reference system of known qualities. U.S. Pat. No. 4,400,089 to Farabaugh discloses a method and apparatus for determining the optical quality of an article by scanning it for thickness, among other properties, through the use of non-contacting "energy beams" which are calibrated by imposing a calibration thickness signal. U.S. Pat. No. 3,531,977 to Chaskelis et al. employs an electronic reference block which electronically simulates the standard test blocks used in the past as a reference. U.S. Pat. No. 4,476,549 to Dragonette et al. relates to calibrating acoustic backscattering instrumentation utilizing a spherical body as a standard target. In U.S. Pat. No. 4,660,419 to Derkacs et al., ultrasonic transducers are calibrated for detecting flaws in pipe or tubular objects by the use of a calibration cylinder having predetermined calibration flaws.
Although the discussed prior art devices have developed effective means for initial calibration and updating, the calibration of energy waves and beam-sensors, the common problem of measurement errors caused by thermal expansion and contraction (thermal stress) have continued to be a serious source of error. In many situations measurements are taken where either the ambient temperature is elevated or the product being measured is in a hot condition. The temperature may not only be extreme but may continuously vary and, although the sensors are originally correctly calibrated, constant error is introduced. Any calibration reference, such as those discussed, which is also subject to the ambient temperature changes would not serve to alleviate this problem.
The problem of distortion due to thermal stress is most pronounced in systems utilizing support frames or "fixtures" for mounting the sensors. The support frame members mounting the sensors are caused to move and distort, introducing error by changing the distance between the sensor head and the material being scanned and/or the distance between the sensors. These fixtures or frames are commonly either O-frames or C-frames, so called because of their configuration. The frame, however, could be any other arrangement of beams which support a sensor or an array of sensors. During thermal expansion, the parallel members of the O-frame or C-frame, which mount the scanning sensor heads, will be displaced sufficiently to displace the heads and will lose their parallelism, introducing error into the measurements.
The present invention seeks to alleviate the problem of thermal expansion by providing a constant reference member for the sensor heads which is itself independent of the influence of ambient temperatures. In this manner, readings may be taken on the reference member or members to obtain a correction signal which is added to the measurement signal to compensate for the effect of thermal stress or other factors that cause distortion of the frame members are displacement of the sensors.